1. Field of the Invention
The present invention relates to a quadrature detecting apparatus for demodulating an angle-modulated signal such as a phase-modulated signal and a frequency-modulated signal, which is adapted to prevent the phase of a demodulated signal from changing even if the amplitude of a balanced angle-modulated input signal is varied.
2. Description of the Prior Art
Conventionally, this type of quadrature detecting apparatus, as disclosed in U.S. Pat. No. 3,667,060 (hereinafter described in connection with FIG. 1), comprises a phase detecting means C (multiplying means), a phase shifting means 2, and first and second coupling means A and B for supplying the input terminals of the phase detecting means C and the phase shifting means 2 with angle-modulated signals, respectively, and is constructed such that when a frequency-modulated signal is demodulated, variations in the amplitude of the frequency-modulated signal will not badly affect the demodulated signal.
Specifically, since the first coupling means A has the phase relationship between input and output signals thereof changing in dependence on the amplitude of an input signal, due to the circuit configuration, an angle-modulated signal, when passing through the first coupling means A, is subjected to phase modulation in accordance with fluctuations in the amplitude thereof, which results in generating components other than the modulated signal in the demodulated signal. However, such a change in phase produced in the first coupling means A is cancelled by providing the second coupling means B constructed identically to the first coupling means A prior to the phase shifting means 2, thereby preventing the influence of fluctuations in amplitude of the frequency-modulated signal on the demodulated output signal.
FIG. 1 illustrates the configuration of a conventional balanced quadrature detecting apparatus. In FIG. 1, reference numeral 1 designates a semi-conductor integrated circuit which includes active elements constituting the quadrature detecting apparatus. Reference letter A designates a first coupling means which is composed of a pair of transistors 107, 108 and a current source 30. Reference letter B designates a second coupling means which is composed of a pair of transistors 101, 102 and the common current source 30. A transistor 603 is a common-base type connection.
Reference letter C designates a phase detecting means which is balanced by two pairs of transistors 109 and 110; 111 and 112. The commonly connected emitters of the respective pairs respectively serve as inputs to the phase detecting means C, while a base-to-base voltage of the respective pairs is used as a control input. Also, the sum of collector currents of the transistors 109, 111 and the sum of collector currents of the transistors 110, 112 are balanced phase detecting output currents.
Reference numeral 2 designates a phase shifting means wherein a frequency-modulated signal is supplied from a phase shift input terminal 6, then a phase proportional to the frequency-modulated signal generated by a resistor 31, inductors 3, 4 and a capacitor 5 is added to the frequency-modulated signal, and the resultant signal is outputted to a phase shift output terminal 7. Reference numeral 8 designates an alternate current ground terminal.
Reference numeral 25 designates a first voltage source, 26 a second voltage source which is set to generate a voltage value smaller than that of the first voltage source 25.
Reference numerals 27 and 28 designate balanced (push-pull type) frequency modulated signal sources which are mixed with a bias voltage source 29. Transistors 19, 21 and 23 constitute emitter followers together with current sources 20, 22 and 24. Pairs of transistors 13 and 14, 15 and 16, 17 and 18 respectively constitute a current mirror. Reference numeral 10 designates a demodulated signal output terminal which is connected with a resistor 11, a voltage source 12 and a capacitor 9 as loads.
The operation of the above-mentioned conventional apparatus will be next described. Referring to FIG. 1, the pairs of transistors 107 and 108; 101 and 102, respectively constituting the first and second coupling means A and B, have their respective emitters commonly connected to the current source 30. Between the bases of the respective pairs, the identical balanced frequency-modulated signal sources 27 and 28 and the bias voltage source 29 are connected so that the two pairs of transistors operate as two sets of amplitude limiting amplifiers. Respective collector currents of the transistors 107 and 108 forming a pair, which are balanced outputs of the amplitude limiting amplifier of the first coupling means A, respectively flow into the common emitters which serve as inputs to the pairs of transistors 109 and 110; 111 and 112 which constitute the phase detecting means C.
One of the collector currents of the transistors 101, 102 forming a pair, which are balanced outputs of the amplitude limiting amplifier of the second coupling means B, is supplied to the phase shift input terminal 6 of the phase shifting means 2 through the transistor 603 which is a common-base type connection, while the other one flows into the second voltage source 26. A signal delivered from the phase shift output terminal 7 through the emitter follower formed by the transistor 19 is connected to one of the base-to-base connections of the pairs of transistors 109 and 110; 111 and 112, while a direct current voltage at the alternate current ground terminal 8 is connected through the emitter follower formed by the transistor 21 to the other one of the base-to-base connections.
In the above-mentioned conventional example, when the amplitude of frequency modulation of the frequency-modulated signals generated from the signal sources 27, 28 are changed, two sets of balanced amplitude limited outputs generated from the two sets of amplitude limiting amplifiers respectively composed of the pairs of transistors 107 and 108; 101 and 102, which also constitute the first and second coupling means, also changes correspondingly. The respective balanced amplitude limited outputs change completely in the same manner. Then, the collector currents of the transistors 107 and 108 constituting the first coupling means A are inputted to the common emitters of the pairs of transistors 109 and 110; 111 and 112, respectively. Also, the collector current of the transistor 101, a component of the second coupling means B, is supplied to the phase shift input terminal 6 through the common-base type transistor 603, whereby an emitter voltage of the transistor 603 hardly changes since the transistor 603 is a common-base type connection.
The phase shifting means 2 adds a phase proportional to the amplitude of frequency modulation of a signal passing therethrough to an output signal, so that the phase of a signal at the phase shift output terminal 7 changes in proportion to the amplitude of frequency modulation of a signal supplied to the phase shift input terminal 6. Therefore, a balanced amplitude-limited output in phase with the signal at the phase shift input terminal 6 is connected to the input, so that balanced modulated output currents proportional to the amplitudes of frequency modulation of the frequency-modulated signals generated from sources 27, 28 are derived at outputs of the phase detecting means C, the control input of which is supplied with a signal at the phase shift output terminal 7 through the emitter follower of the transistor 19. Then, one of the demodulated output currents balanced by the three pairs of transistors 17 and 18; 13 and 14; 15 and 16 has its direction changed, whereby the demodulated output currents are eventually converted to a balanced demodulated signal current proportional to the amplitudes of frequency modulation of the frequency-modulated signals generated from the sources 27, 28 at the demodulated signal output terminal 10. The balanced demodulated signal current can be taken out as a demodulated signal voltage by the resistor 11, the voltage source 12 and the capacitor 9.
Even if the amplitudes of the frequency-modulated signals generated from the frequency-modulated signal sources 27, 28 change to cause fluctuations in the phase relationship between the input and output of the amplitude limiting amplifier composed of the pair of transistors 107 and 108 which also constitute the first coupling means A, the phase of the output from the amplitude limiting amplifier composed of the pair of transistors 101 and 102 also constituting the second coupling means B in a configuration substantially equal to the first coupling means A also fluctuates in a similar manner.
The fluctuation in phase between the input and the control input of the phase detecting means C is thus cancelled, which results in eliminating the phase fluctuations therebetween and accordingly preventing the demodulated output from being influenced by such fluctuations.
As described above, the conventional quadrature detecting apparatus can also prevent a demodulated output from being influenced by fluctuations in amplitude of frequency-modulated signals generated from signal sources.
However in the above-mentioned conventional quadrature detecting apparatus, if the amplitude of a driving voltage fed to the second input of the phase detecting means C is increased to make larger the amplitude of the control input supplied to the phase detecting means C, for the purpose of enhancing the modulation sensitivity and reducing noise possibly produced during demodulation, fluctuations in the input and output of the phase detecting means C caused by fluctuations in the amplitudes of the frequency-modulated signals generated by the frequency-modulated signal sources 27, 28 connected to the amplitude limiting amplifiers constituting the first and second coupling means A, B do not become equal to each other, whereby such fluctuations appear in a demodulated output.
More specifically, as long as the amplitudes of the base-to-base voltages in the phase detecting means C composed of the pairs of transistors 109 and 110; 111 and 112, respectively having their emitters commonly connected, are small, there are also small changes in the voltages at the respective common emitters due to changes in the amplitudes. However, if the amplitudes of the base-to-base voltages are made larger, changes in voltages at the common emitters also become larger, which results in largely changing the base-to-collector voltages of the pair of transistors 107 and 108 of the amplitude limiting amplifier also constituting the first coupling means A. On the other hand, the base-to-collector voltages of the pair of transistors 101 and 102 of the amplitude limiting amplifier also constituting the second coupling means B hardly change because of the circuit configuration.
Therefore, fluctuations in the amplitudes of signals generated from the frequency-modulated signal sources 27, 28 cause fluctuations in the amplitude of the base-to-base voltage, whereby the relative phases of the two sets of balanced amplitude limited outputs from the amplitude limiting amplifiers constituting the first and second coupling means A and B which are respectively composed of transistors so as not to present the same base-to-collector voltages are subjected to fluctuations, which results in failing to achieve the original object.